The present invention relates to a method and apparatus for liberating and removing gas bubbles entrained in drilling fluid, such as drilling mud, utilized in the drilling operation of wells, such as oil wells.
In conventional well drilling operations it is customary to pump substantial quantities of a specially formulated drilling mud down through the drill stem to be discharged through apertures adjacent the drill bit. This drilling mud serves not only to lubricate the drill bit and to remove the displaced rock, sand and other materials from adjacent the bit to a discharge point on the surface of the well site, but also to prevent the escape or blowout of natural gas encountered by the drill. During the flow of the mud from adjacent the drill bit to the surface it frequently becomes contaminated in the well with natural gas or air, this contamination taking the form of entrained minute bubbles of gas in the mud. For economic reasons it is desirable to recycle the special drilling mud through the well instead of continuously supplying new material. However, the presence of the entrained gas bubbles in the drilling mud decreases its density and substantially diminishes its efficiency both in lubricating the drill and in preventing well blowouts. Accordingly, it is desirable to degas the drilling mud to prepare it for recycling through the well.
Because of the physical characteristics of drilling mud, particularly its abrasiveness and its high viscosity which makes liberation of entrained gas bubbles difficult, special degassing systems have been required. Drilling mud degassing apparatus and procedures have generally involved centrifugal pumps or other rotary impellers in order to obtain the necessary agitation of the viscous liquid. Conventional reciprocating cavitators, such as disclosed in Baker, U.S. Pat. No. 2,376,221, for use with liquid food products such as fruit juices, have generally proved unsuitable for the removal of minute entrained gas bubbles from viscous drilling mud. One cause of the inadequacy of such reciprocating agitators in drilling mud applications is the tendency of the highly viscous liquid to flow around the reciprocating agitator without substantial mixing action, such that gas may be removed from the mud immediately adjacent the agitator, while little degassing is effected of the liquid spaced from the agitator, such as that adjacent the conduit walls. Accordingly, such prior art reciprocating cavitators have generally been found unsuited for applications involving such viscous materials.